Low noise traveling-wave tube



1959 M. R. CURRIE LOW norss TRAVELING-WAVE TUBE Filed Dec.

INVENTOR. Malcolm R.Currie,

By 4 5m.

United States Patent 2,859,021 Patented Jan. 13, 1 959 LOW NOISETRAVELING-WAVE TUBE Malcolm n. Currie, Beverly Hills,

Hughes Aircraft Company, poration of Delaware Califi, assignor to CulverCity, Calif., a cor- This invention relates generally to microwavetubes, and more particularly to a traveling-wave tube having a low noiseelectron gun.

In many applications of electronic amplifiers, the amplifying device,though it have an effective magnitude of amplification, is limited inits usefulness by its inherent internal noise generation. This is truein traveling-wave tubes, as well as in electron tube devices in general.However, the problem is a particularly acute one in the traveling-wavetube field because very high amplifications are achieved in a singletube with a relatively high power beam of electrons; and it is anexceedingly complex problem to generate, project and focus awell-defined, high perveance, stream of electrons which has a low levelof radio-frequency noise associated with it. A further and even moretroublesome problem presents itself when it is attempted to provide ahollow electron beam such as is desirable in most backward-wavetraveling-wave tubes. A complex set of focusing and acceleratingelectrodes must be arrenged about the meam in the electron gun region ofthe'tube. Because the beam is hollow, it has been found to be desirableto employ with each of several of the electrodes an associated centerdiscdisposed within the hollow electron beam for providing betterformation and smoother acceleration of the beam and to keep potentialdistributions across the beam as flat as poss'ible. -It has been thepractice in the past to support the center discs by means of thin gridwires which extend radially between the discs and the associated outerelectrode. Such configurations are in general satisfactory; however, inapplications Where an extremely low noise figure associated. with theelectron stream is desired, the grid wires, because they inherentlyintercept electrons, cause an appreciable contribution to the totalnoise figure of the tube, such as in fact to cause the tube to beinoperable in such an application.

it is, therefore, an object of the present invention to provide a hollowbeam traveling-wave tube having an extremely low noise figure.

it is another object to provide such a traveling-wave tube in which thecenter discs in the electron gun electrode arrangement need not besupported by means which intercept stream electrons.

These and other objects are achieved in the present invention bysupporting the center discs for a hollow beam electron gun by a rigidcenter column supported through the nonemissive center portion of thecathode. The cen ter column also carries wires or other conductors backthrough the cathode so that each center portion or disc may beelectrically connected to its respective outer portion.

These and other novel features which are uniquely character c of thisinvention, both as to its structure and operation, together with furtherobjects and advantages, will become apparent to one skilled in thetraveling-Wave tube art from the following detailed description and anddrawing.

In the drawing:

Pig. 1 is a schematic view, partially in section, of a backward-wavetube constructed in accordance with the present invention; and

Fig. 2 is a more detailed cutaway view, on an enlarged scale, of anelectron gun as utilized by the present invention.

Referring with more particularity to the drawing, Fig. 1 depicts abackwardwave amplifier tube 10 as an example of a hollow beam device,constructed in accordance with the present invention. An evacuated glassenvelope 12 houses a slow-wave structure 14, illustrated here as asingle helix and extending along a major portion of the length of thetube, and an electron gun 16 within its enlarged'left hand extremity.Electron gun. 16 projects a hollow stream of electrons toward the rightthrough the slow-wave structure 14 which eventually impinges upon theinner surface of a collector electrode 18 disposed at the right-handextremity of envelope 12. In the particular example chosen, the streamcurrent and other parameters are adjusted such that wavesofelectromagnetic energy are amplified along the length of slowwavestructure 14. An output waveguideZgtl is electromagnetically coupled ina conventional fashion to the output end of slow-wave structure 14 by apickup lead 22 which is connected between slow-wave structure 14 and amatching ferrule 24.

Similarly, at the collector end of tube 10, an input waveguide 21 iscoupled to that end of slow-wave structure 14 via a pickup lead 23 whichis connected between an input matching ferrule 25 and slow-wavestructure 14. Electron gun 16 includes an annular cathode 26 heated asby a filament 28 causing the emission frointhe cathode of a suitablecurrent of electrons. A beam-focusing electrode 30, having an outerportion in the shape of a hollow truncated cone and an inner portiondescribed below, is electrically connected to cathode28 and aids informing a beam of electrons whichis accelerated by a first anode 32 andthence by a subsequent anode 34. Electrode 3t and anodes 32 and 34 aresupported by rods 35 to the basexend of tube 10. Projecting along/theaxis of the tube through the center of cathode 28 is a rigid dielectricrod 36 which supports a center portion 3 6 of focusing electrode 30;Center portion 30' is the shape substantially of a small cone. Furtheralong rod 36, and in line with accelerating electrode 32, there issupported on the rod an inner portion, or center disc, 32, and similarlyfurther along the rod, and coplanar with electrode 34, there issupported its inner portion, or disc, The radial dimension of theannular space between respectively electrodes 34]) and 30', 32 and 32'and 34 and is just sulficient to allow passage of the desired hollowelectron beam with no interference or interruption therewith.

A voltage source 38, having its positive end grounded, is tapped atvarious points which are connected in conventional manner to cathodeheater 28, cathode 26and accelerating electrodes 32 and 34. A voltagesource 40 is connected between collector electrode 18 and ground, and isof a positive magnitude such that secondarily emitted electrons from thecollector will be held or returned to the collector and not be allowedto traverse any portion of slow-wave structure. 14. Attached to (11electric rod 36am conductors, such as wires, forelcctrically connectingelectrode Sit-to 3t), 32 and. 32 and 34- and M. Such wires pass backalong rod 36 and are. externally connected to the respectiveouterportions of the electrodes. In this manner, although theinnerportions are electrically connected to the outer portions, no wiresinterfere or interrupt the electron stream.

A solenoid 42 substantially surrounds traveling-wave} tube and producesan axial magnetic field to aid in constraining and confining theelectron stream into the desired hollow form. Solenoid 42 is shownenergized by electric source 44. a

A practical embodiment ofa low noise electron gun 46 is shown in Fig.2which illustrates .a typical electron gun to be utilized in accordancewith the present invention. A nonconductive base support 48, which maybe the base end of envelope 12, would be supported by envelope 12 (nototherwise shown in Fig. 2) and supports and holdsin alignment thevarious electrodes of gun 46. These include cathode 26 and acceleratingand beam-forming electrodes 50, 52,54, 56 and 58. Cathode 26is securedto base 48 by a hollow cylindrical support 6t), and electrodes 50, 52,54, 56 and 58 are each secured to base 48 by means of a series of axialdielectric rods 62 disposed about the periphery of base 48. A dielectricrod 64, extending along the axis of gun 46 and supported by base 48,similar to rod 36 of Fig. l, secures and supports, coplanarly,respective center portions, or inner discs, of electrodes 50, 52, 54, 56and 58. These center portions are designated by primed numeralscorresponding to those of the outer portion of each electrode. The innerportions of the electrodes are electrically connected to theirrespective outer portions by means of fine conductive wires, such as 66and 68, each of which are insulated from each other and fro-m the otherelectrodes by means of lengths of quartz capillary tubes, such as 70 and72. There would of course be one such wire for each electrode, althoughonly two are shown in the figure. Voltage source 38 is tapped in thisexample at six diiferent points to provide seven different potentials,the most negative of Which-is designated as zero, and the most positiveof which is designated by V Cathode 26 and one side of filament 28 areconnected to the zero point; the other end of filament 28 is connectedto the V point; electrode 52 and its inner portion'52 are connected tothe V point; electrode 50 is similarly connected to the V point;electrode 54 is connected to the V point; electrode 56 is connected tothe V point; and electrode 58 is connected to the V point. An aquadagcoating 68 is placed on the interior surface of the neck of envelope 12,as shown in Fig. 1, and is also connected to the V point, as wouldnormally be matching ferrule 24 and slow-wave structure 14.

It is to be noted that electrode 52 is at a more 'negative potentialthan is electrode 50. The reasons for this and the exact structure ofthis portion of the electron gun are disclosed and claimed in'aco-pending application Serial No. 631,129 by Malcolm R. Currie andDonald C. Forster, which invention is assigned to the assignee of thepresent invention and filed concurrently herewith. It is'tobe stressedthat the present invention resides in the manner of supporting the innerportions of the various disc electrodes and the combination thereof witha low noise traveling-wave tube. V

In operation it may be seen that the hollow beam emitted by cathode 26may traverse the entire electron gun wtih no interruption orinterference, as by grid wires which are conventionally used to supportthe inner portions of the various" electrodes. Such conventionalsupports severely limit the available minimum noise figure in a lownoise traveling-wave tube, even though satisfactory in otherapplications where a low noise figure in the tube is not critical.

What has here been disclosed are two embodiments of the presentinvention, viz., those shown in Fig. 1 and Fig. 2. The embodiment ofFig. 2 includes a more complexarrangement of electrodes, while Fig. l isa simplified version submitted for purposes of explanation. Obviously,the degree of complexity of electrode arrangement, as well as thestructureof the accompanying traveling-wavetube; maybevaried' withinglarge range:

What is claimed is:

1. In a traveling-wave tube, a low-noise hollow beam electron guncomprising: a: mounting baseplate extending across the electron gun endof said traveling-wave tube substantially in a plane perpendicular tothe axis thereof; an annular cathode having an axial passageway andsupported symmetrically with respect to said axis on said baseplate; aninner rigid dielectric supporting rod attached to said baseplate andextending axially along said traveling-wave tube from the electron gunend thereof through and beyond said passageway in said annular cathode;a plurality of outer dielectric supporting rods spaced about theperiphery of said cathode and attached to said baseplate; a plurality ofbeam-forming and accelerating'electrodes disposed perpendicularly tosaid axis, each comprising an outer annular member for surrounding saidhollow beam and a circular inner memberdisposed within said hollow beam,said outer annular member being supported by said outer supporting rods,and said inner members being supported coplanarly by said innerdielectric supporting rod; and a plurality of electrical conductor meansconnecting each of said inner members to respective ones of said outermembers, each of said conductor means being connected to one of saidinner members and extending along said inner supporting rod through saidbaseplate and thence to an associated outer member, said inner membersbeing thus supported on said inner rod in a manner such that said hollowbeam passes between respective associated members of each electrodewithout interference or interceptio-n thereby, whereby an exceedinglylow-noise hollow electron beam is produced.

2. in a backward-wave traveling-wave tube, a lownoise hollow beamelectron gun comprising: a mounting baseplate extending across theelectron gun end of said traveling-Wave tube substantially in a planeperpendicular to the axis thereof; an annular cathode having an axialpassageway and supported symmetrically with respect to said axis on saidbaseplate; an inner rigid dielectric sup porting rod attached to saidbaseplate and extending axially along said traveling-wave tube from theelectron gun end thereof through and beyond said passageway in saidannular cathode; a plurality of outer dielectric supporting rodsattached to said baseplate and spaced about the periphery of saidcathode; a plurality of profile-shaping and accelerating electrodesdisposed perpendicularly to said axis, each comprising an outer annularmember for surrounding said hollow beam and a circular inner memberdisposed within said hollow beam, said outer annular member beingsupported by said outer supporting rods, and said inner members beingsupported coplanarly by said inner dielectric supporting rod; and-aplurality of electrical conductor means connecting each of said innermembers to respective ones of said outer members, each of said conductormeans being connected to one of said inner members and extending alongsaid inner supporting rod through said baseplate and thence to anassociated outer member, said inner members being thus supported on saidinner rod in a manner such that said hollow beam passes betweenrespective associated members of each electrode without interference orinterception thereby, whereby an exceedingly low-noise hollow electronbeam is produced. i

3. In a backward-wave traveling-wave amplifier tube, a low-noise hollowbeam electron gun comprising: a mounting baseplate extending across theelectron gun end of said traveling-wave tube substantially in a planeperpendicular to the axis thereof; an annular cathode having an axialpassageway and supported symmetrically with respect to said axis on saidbaseplate; an inner rigid'dielectric supporting rod attached to saidbaseplate and extending axially along said traveling-wave tube from theelectron gun end thereof through and beyond said passageway in saidannular cathode; a plurality of outer dielectric supporting rodsattached to said baseplate and'spac'ed" about the periphery of saidcathode; a plurality of profileshaping and accelerating electrodesdisposed perpendicularly to said axis, each comprising an outer annularmember for surrounding said hollow beam and a circular inner memberdisposed within said hollow beam, said outer annular member beingsupported by said outer supporting rods, and said inner members beingsupported coplanarly by said inner dielectric supporting rod; and aplurality of electrical conductor means connecting each of said innermembers to respective ones of said outer members, each of said conductormeans being connected to one of said inner members and extending alongsaid inner supporting rod through said baseplate and thence to anassociated outer member, said inner members being thus supported on saidinner rod in a manner such that said hollow beam passes betweenrespective associated members of each electrode without interference orinterception thereby, whereby an exceedingly low-noise hollow electronbeam is produced.

4. In a beam type electron tube, a hollow electron beam gun comprising:mounting means extending across at least a portion of the electron gunand of said electron tube; an annular cathode having an axial passagewayand supported concentrically with respect to the axis of said tube fromsaid mounting means; an inner rigid dielectric sup porting rod supportedfrom said mounting means and extending axially along said electron tubefrom the electron gun end thereof through and beyond said passageway insaid annular cathode; a plurality of critically disposed beam formingand accelerating electrodes spaced along and concentrically about saidaxis, each comprising an outer annular member for contiguouslysurrounding said hollow beam and a circular inner member disposedsubstantially co-planarly with said outer annular member contiguouslywithin said hollow beam, said inner coplanar members being supported bysaid inner dielectric supporting rod in axial sequence therealong; and aplurality of electrical conductor means connecting each of said innermembers to respective ones of said outer members, each of said conductormeans being connected to one of said inner members and extending alongsaid inner supporting rod to said mounting means and thence to itsassociated outer member, said hollow beam passing through the annularspace between the inner and outer respective associated members of eachelectrode without interception or interference thereby, whereby anexceedingly low noise hollow electron beam is produced.

References Cited in the file of this patent UNITED STATES PATENTS2,409,224 Samuel Oct. 15, 1946

